Existing intraoral dental image sensors are not always optimized from the standpoint of compactness owing to the fact that it is necessary to find compromises between fabrication cost and the desired optical performance, and that these compromises must be made to the detriment of other parameters, such as size, whether this be their length, their width or their thickness. Now, size is an essential parameter since the image sensor has to be introduced into a patient's mouth. Each millimeter or fraction of a millimeter saved, in each of the three dimensions, for a given image acquisition surface, represents a significant progress.
Existing radiological image sensors are usually produced in the following manner: a series of individual, visible image detection chips on a silicon wafer by a number of operations in which insulating, conducting and semiconductor layers are deposited and etched, and doping operations, etc., all these operations being carried out starting from a front face of the wafer. After collective fabrication on the wafer, the wafer is diced into individual chips, and the chips are each mounted on a support. Conducting areas on the chip are connected to conducting areas on the support by wires using the technique of wire bonding. Next, in the case of radiology, a scintillator is deposited (in general bonded) to the front face of the silicon chip. The scintillator is designed to convert the incident X-rays into visible radiation at a wavelength at which the image sensor is sensitive.
It has already been proposed to interpose, between the scintillator and the silicon chip, a flat fiber-optic plate having a thickness of a few microns, the purpose of which is to attenuate (by simple absorption) the residual X-radiation that has passed through the scintillator without being converted into visible light. This is because the residual X-rays tend to be generated in the silicon from parasitic electrons that represent more noise than the useful signal, and it is desirable to eliminate them. The fiber plate attenuates the X-rays, but it does not attenuate the visible light that has been emitted by the scintillator. The resolution of the visible image generated in the scintillator is preserved by the fiber-optic plate and is repeated on the sensitive surface of the sensor—this is its great advantage.
In this case, it is not a simple scintillator wafer that is bonded to the silicon chip but a fiber-optic plate coated with a scintillating layer. The scintillating layer may be a layer of cesium iodide deposited on the plate, or else, more usually, it is a carbon film coated with cesium iodide that is bonded to the plate by a transparent adhesive interposed between the cesium iodide and the plate. Materials other than cesium iodide may be used (sheets of plastic filled with gadolinium oxysulfide, etc.).
The devices fabricated up until now are not optimized in terms of the dimensions of the image sensor.